1. Field of the Invention
This invention relates to smoke meters and, particular, to metering devices for determining the level of smoke content in, for example, the exhausts of road vehicles.
2. Description of the Art
The emission of quantities of smoke from vehicle engines has become environmentally unacceptable. In order to control the level of smoke emission it is necessary to obtain some indication as to the quantity of smoke being emitted. Such metering is necessary before any control can be exercised over vehicles which emit too much smoke.
The elements in the exhaust emissions which the present application addresses include carbon particles and other particulate material which may be present. The presence of water particles is not normally of particular concern and there are other means for determining the presence of noxious gases.
In metering the levels of smoke in vehicle emissions it has been proposed to pass a light beam through a sample of exhaust gas and to detect the amount of light which is able to be transmitted from the light source to a light detector. However, deposits of materials from the smoke sample on the surfaces of the light emitting and the light detecting means can affect the amount of light passing through the sample and one object of the present invention is to reduce such deposition.
In British Patent Specification No. 1135146 and in European Patent Specification No. 0456202 there are described smoke meters in which an attempt has been made to keep optical surfaces clear of deposits by using air flows. In European Patent 370248 deposits on the optical surfaces are burned off.